They are the same. 'after' is the reason.Originally Posted by blacknomi
Both 'after' and the Perfect mark the order of the events, but 'after' is required as a subordinating conjunction, whereas the Perfect 'have -ed' is not required because the meaning it expresses is carried by the subordinator, and given that semantic redundancy, some speakers will omit the Perfect marking, notably 'had', which in the case of an irregular verb like 'leave, left' also involves changing to irregular form 'left' back to its regular form 'leave', and go with the Simple Present:
Present Perfect: You will have time after they have left.
Simple Present: You will have time after they leave.